Method of and system for measuring phase shift



. July 27, 1954 D. A. ALSBERG METHOD OF AND SYSTEM FOR MEASURING PHASE SHIFT Original Filed Dec. 14, 1948 7 Shets-Sheet l ATTORNEY July 27, 1954 D. A. ALSBERG 2,685,063

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METHOD OF AND SYSTEM FOR MEASURING PHASE SHIFT v Iignal Filed Dec. 14, 1948 7 Sheets-Sheet 3 DIFFERE N TML DETECTOR MNITORING Nl/ ENTOR D. A. ALSBERG BV Oar CM2@ ATTORNEY July 27, 1954 METHOD OF AND SYSTEM FOR MEASURING PHASE SHIFT Original Filed Dec. 14, 1948 D. A. ALSBERG '7 Sheets-Sheet 4 A 7 TORNE V July 2.7, 1954 D. A. ALsBERG METHOD OF AND SYSTEM FOR MEASURING PHASE SHIFT 7 Sheets-Sheet 5 Original Filed Dec. 14, 1948 NVENTOR D. LSBERG By ATTORNEY July 27, 1954 D. A. ALSBERG 2,685,053

METHOD OF AND SYSTEM FOR MEASURING PHASE SHIFT Original Filed Deo. 14, 1948 7 Sheets-Sheet 6 @Y @w @Sth ATTORNEY July 27, 1954 Original Filed Dec. 14, 1948 D. A. ALSBERG METHOD OF AND SYSTEM FOR MEASURING PHASE SHIFT 7 Sheets-Sheet 7 FOUR TERM/NAL NETWORK /NVE/v TOR D. A. ALSBERG @Y @mi ATTORNEV Patented `luly 27, 1954 UNITED STATES PATENT QE'FICE METHOD F AND SYSTEM FOR MEASURING PHASE SHIFT Dietrich A. Alsberg, Berkeley Heights, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York 22 Claims.

This invention relates to a method of and system for measuring the phase shift and 'the transmission of electrical apparatus, and more specifically to a movable index for use in such measuring method and system. While the invention is described in connection with measuring apparatus, it has other applications as will be hereinafter mentioned.

This application is a division of the application of D. A. Alsberg-R. P. Muhlsteff, Serial No. 65,208, led December 14, 1948, now Patent No.

2,622,127, issued December 16, 1952.

One known type of circuit for measuring the insertion phase shift of an apparatus under test involved the procedure of (ci) firstly, substituting a short circuit for the apparatus under test and then adjusting a calibrated phase shifter of the circuit until a zero reading was provided on a suitable indicator whereupon the phase shifter would show A degrees; and secondly, replacing the short circuit with the apparatus under test and then adjusting the calibrated phase shifter until the zero reading was reestablished on the indicator whereupon the phase shifter would now read B degrees. The insertion phase shift of the apparatus under 'tc-st Would then be (B-A) degrees. Such subtraction was cumbersome and tended toward algebraic errors on the part of the operating personnel. Also, in the past, one phase shifter was used to provide the A reading, and therefore a second phase shifter was required to establish a direct indication of the (Bee.) reading.

Now, let it be assumed that there has been devised a perfect phase shifter calibrated with a linear scale variable continuously through 360 degrees by rotation of a shaft. Such line-ar scale could be coaxially mounted with 'the shaft and adjusted position independently thereof. The initial zero balance of the system could then be established by moving the shaft of the phase shifter through A degrees, and thereafter rotating the linear scale until its Zero point correspends exactly with a iixed index serving as a reference point. Thereafter, when the circuitl with the assumed phase shifter linear scale is utilized to measure the insertion phase shift of the apparatus under test, the linear scale will directly indicate such phase shift, i. e., the former (B1-A) reading. As a practical matter, hou'f'ever, a perfectly linear phase shifter has not yet been devised. A residual error which can be expressed as the deviation from perfect linearity is always present in the phase shift calibration. If such deviation were incorporated in the scale, the latter would become non-linear. As a consequence, it would be no longer possible to move lthe scale relative to the phase shifter shaft, as

such movement would destroy the calibration.

The present invention contemplates a directreading system with linear scales for measuring phase shift continuously 'through 366 degrees by utilizing a movable index for automatically compensating such system for deviations from linearity.

The main object of the invention is to provide a system for directly indicating phase measurements.

Another object is to provide a movable index for a direct-reading phase measuring system having linear scales.

A further object is to provide a direct-reading phase measuring system with an over-al1 zero at which the measuring scales thereof are ad- ,instable to zero to represent the over-all system zero.

A further object is to derive a primary standard of phase shift which is submultiple of 360 degrees.

Another object is to provide a direct-reading phase measuring system with a fixed correction curve for residual error but. with scales adjustable relative to the fixed correction curve.

Another object is to provide calibrated apparatus with a movable index for indicating its deviation from a predetermined scale.

Another object is to simplify the utilization of moving indices with calibrated apparatus.

in a specific embodiment, the invention comprises a photographic film containing a correction curve for the `residual error of electrical measuring,apparatus,l say, for example, a calibrated phase shifter comprising a condenser adjustable continuously through 369 degrees by the movement of a rotor controlled by the rotation of a shaft. The nlm is mounted in a predetermined position on the periphery of a transparent drum which is iixedly mounted coaxially With the phase shifter shaft.` A beam light passed through the drum and a portion of the correction curve thereon is projected onto a screen disposed adjacent a linearly calibrated line scale connected to the shaft and representing one division ci a second linearly calibrated coarse scale mounted coaxially with the shaft. The projected beam or" light moves hack and forth on vthe screen to the right or left oi a central point' amounts corresponding to residual error of the phase measuring apparatus at different positions of its rotor through the 350 degrees shift effected thereby, such central point representing no residual error. The fine scale read against the light projection on the screen which constitutes a movable index therefor.

A feature concerns the use oi a movable scale with a calibrated oscillator or other calibrated apparatus. Another feature involves the establishment of a primary standard of phase shift,

which is a submultiple of 360 degrees, and its transfer to apparatus to be calibrated.

A feature includes the provision of a directreading phase measuring system having linear Scales and an automatic compensation for the residua-l error thereof. Another feature is that a change of calibration necessitates only a removal of the old film containing the correction curve for enabling the automatic compensation for the residual error of the system and the replacement thereof with a'new film'bearing the new correction curve. Another feature concerns the initial establishment ofafixe'd relationship between the correction curve and rotor of the phase shifter whereby the coarse and fine scales thereof may be adjusted to different relativepo'si'- tions therebetween without disturbing the initial relationship of the correction curve and phase shifter. Another feature involves the establishment of a zero condition in the over-all phase measuring system whereat the coarse and fine scales of the phase shifter can be adjusted to zero reading without adversely affecting the relationship between the correction curve and phase shifter. A further feature is that the directreading aspect reduces the tendency of operating personnel to make errors in the algebraic computations required by prior art phase measuring systems.

The invention will be readily understood from the following description when taken together with the accompanying drawing in which:

Fig. 1 is a box diagram of a system for measuring the transmission and phase characteristics of an unknown apparatus;

Fig.- 1A is a circuit modification that may be substituted in Fig. 1;

Fig. 2 is a box diagram showing a conventional arrangement for Calibrating, with reference to a standard phase shifter, a phase shifter used in Fig. 1;

Fig. 3 is a box diagram showing an arrangement for Calibrating, with reference to an absolute standard of phase, a phase shifter used in Fig. 1;

Fig. 4 is a schematic circuit showing a fundamental concept of the differential phase detector used in Fig. 1;

Figs. 5 and 6 are vector diagrams of certain phase relationships obtainable in Fig. 4;

Fig. '7 is a schematic circuit showing the basic type of phase detector used in Fig. 1;

Fig. 8 is a modification of Fig. '7 and is a schematic circuit showing the differential detectors for measuring transmission and phase in Fig. 1;

Fig. 9 is a schematic circuit of an arrangement for providing the phase and transmission indicators in Figs. 1, 7 and 8, with coarse and fine scales;

Fig. 10 illustrates the dial arrangement for the calibrated attenuator used in Fig. 1;

Fig. 11 is a schematic circuit showing an illuminating arrangement used for measuring gain and/or loss in Fig. 1;

Figs. 12, 13 and 14 delineate mechanical arrangements of an optical cam associated with a phase shifter in Fig. 1;

Fig. 15 is a modification of the arrangement shown in Fig. 12;

Fig. 16 is an alternate embodiment of the optical system shown in Fig. l2 and usable in Fig. 1;

Fig. 1'7 shows an optical system associated with a one scale measuring system;

Fig. 18 is another embodiment of "an optical system usable in Fig. 2; and Fig. 19 is schematic circuits defining measurements obtainable with Fig. 1.

In'the following description identical reference numerals are utilized tc identify the same elements appearing in the several figures of the drawing.

OVER-ALLV NIEASURING CIRCUIT Fig. 1 illustrates a system for measuring insertion phase shift and transmission through unknown apparatus under test I0, and may be modified by substituting Fig. 1A to the left of line 2.-2 in Fig. 1 for measuring transfer transmission and transfer phase shift. Insertion phase shift and transfer phase shift difference may be defined by referring to Fig. 19 in which Fig. 19A comprises a source of voltage E having an impedance Z1 and connected to load Zz across which is a voltage E1; and Fig. 19B comprises a source of voltage E having an impedance Z1 and connected to the input terminals of a generalized four-terminal network whose output terminals are connected to load Z2 across which is Voltage E2. A voltage E3 occurs across the input terminals of the four-terminal network; and a Voltage E4 appears across the output terminals of the latter network. Insertion loss and gain is defined as the ratio of voltage E1 to voltage E2 in Figs. 19A and B; insertion phase shift is defined as the difference in phase between voltages E1 and E2 in Figs. 19A and B. Transfer transmission is defined as the ratio of voltage E3 to voltage E4; and transfer phase difference is defined as the difference in phase shift between voltages E3 and E4.

In the system of Fig'. l, an oscillator lI of well-known design supplies (l) a testing signal f of a frequency varying from 50 kilocycles to 3,600 kilocycles through an attenuator I2 to a resistance splitting pad I3; (2) a second signal f1 having a fixed frequency of 15,000 kilocycles to a synchronized oscillator I4; and 3) a third signal f2 varying in frequency from 11,400 kilocycles to 14,950 kilocycles to one input of modulator I5. The oscillator I4 also supplies to a second input of modulator I5 a signal f3 having a fixed frequency of 15,031 kilocycles as disclosed in the copending application of D. Leed, Serial No. 65,130, filed December 14, 1948.

The splitting pad I3 is so arranged as to divide the testing signal f into two equal voltage portions of which one portion is supplied through the apparatus under test I0 to the input of a modulator I6 while the second portion is supplied directly to a modulator Il, both modulators being of a familiar design. The second inputs of modulators I6 and II are also supplied from the output of modulator I5 and through an attenuator I8 with waves f4 whose frequencies vary from 81 kilocycles to 3,631 kilocycles. The output of modulator I6 is supplied through a calibrated attenuator I 9 to the unknown input, as the test signal of the unknown branch, of a differential detector 20 comprising a differential phase detector 20a Whose output is connected to a phase indicator 2| and a differential transmission detector 20h whose output is connected to a differential transmission indicator 25 for effecting phase and transmission measurements, respectively, of the apparatus under test III.

The output of modulator I'I is supplied through attenuator 22, calibrated phase shifter 23 to the standard input, as the test signal of the standard path, of the differential detector 20. An AVC connection extends from the output of differential. transmission detector 2th, Figs. l and 8, to modulator l5, Fig. l, for a purpose that will be hereinafter mentioned. The differential detector and indicators 2! and 25 will also be hereinafter explained in connection with Figs. '7, 8 and 9. A mechanical connection 32 extends between attenuators I2 and 22, and an electrical connection 33 extends between attenuators l2 and I9 for purposes that will appear hereinafter.

CALIBRATION OF PHASE SHIFTER 23 IN FIG. 1

For the purpose of achieving a measurement of the insertion phase shift of the apparatus under test lll in Fig. l, there is employed a phase shifter 23 of the well-known four-quadrant type having a rotor for varying phase continuously through 360 degrees as illustrated in Fig. l2. In this connection it will be understood that other well-known types of phase shifters could be alternately utilized. As a perfectly linear phase shifter is normally unobtainable, it is necessary to calibrate the phase shifter 23 in order t0 note the deviations from linearity through 360 degrees as occasioned by residual error there- 1n.

In this connection, an elementary circuit shown in Fig. 2 for Calibrating phase shifter 23 will be initially considered. Referring to Fig. 2, a generator 2? of signal waves having a predetermined frequency is connected to a splitting resistance pad 28 from whose output one portion is applied through a phase shift standard 29 to the standard input of a differential phase detector Zlia. The other output portion from splitting pad 28 is applied through phase shifter 23, which is to be calibrated, to the unknown input of the differential detector 2da. A suitable indicator 2l connected to the output of differential phase detector 20a measures the difference between the vectorial sum and difference of the standard and unknown voltages applied to the input thereof as will be described in detail hereinafter. Thus, the phase shifter 23 may be calibrated in an obvious manner by comparison with the standard phase shifter 29 in order to ascertain the deviation from linearity of the phase shifter 23 at each point in the calibration through 360 degrees.

Heretofore, the phase shift standard 2Q in Fig. 2 was dened by l, measuring the reactance and the resistance of a standard network and computing phase shift from such measurements; 2, using the sum and difference method of phase measurement which defines phase shift in terms of voltage ratios; 3, utilising a measurement of portions of a wavelength such as possible with Lecher wires; and ll, pulse measuring devices. A difficulty with all such prior arrangements was that inaccurate measurements tended to result as the measuring frequency increased.

Instead of using a derived standard phase shifter 29 as in Fig. 2, an absolute standard of phase may be determined by use of the circuit shown in Fig. 3. This determination is absolute or primary in the sense that phase shift is dened directly without reliance on external standards defined by frequency, or reactance, etc., such, for example, as in the case of the standard phase shifter 2'9 in Fig. 2.

The arrangement of Fig. 3 about to be described is not limited by frequency, and is equally effective at a frequency of l0 cycles as well as at the frequency of 1,000 megacycles or higher. To effect the phase determination of Fig. 3, it is only necessary that the frequency of the testing oscillator be substantially stable; that the detector, auxiliary networks and the apparatus to be calibrated be sufficiently stable; and that the error due to cross-talk and pick-up in the system involved be sufficiently small.

The basic methodV of Fig. 3 is one of successive approximation and is most readily comparable to the problem of subdividing a circle with a compass. Let it be supposed that the circumference of a circle is to be subdivided into three equal arcs, each of degrees. First, a starting point on the circumference of the circle is arbitrarily selected and then the compass is adjusted to span 120 degrees. Commencing at the arbitrary starting point, three steps are measured oif on the circumference of the circle with the compass. Assuming the compass spans exactly 120 degrees, the third step would return to the arbitrary starting point. If not, the spread of the compass is changed and the attempt to subdivide the circumference of the circle into the three equal arcs is repeated in a series of approximations until finally the arbitrary starting point is reached. Then, each 1Z0-degree arc is divided into three equal arcs, each of 40 degrees. Thus, effecting the subdivisions by a factor 3 tends to reduce the cumulative error which would tend to be present if the subdivision of the circumference were attempted by a higher factor, say 9, for example.

In the circuit of Fig. 3, a generator 34 of signals having a predetermined frequency is connected through a splitting pad 35 having one terminal connected to a phase shifter 23 to be calibrated and its opposite terminal connected to a continuously variable uncalibrated phase shifter 3l. The phase Shifters 23 and 37 are continuously adjustable through 360 degrees, the former having its output applied through a variable attenuator 38 to one input of differential phase detector 2Go; whose output is connected to phase indicator 2|. The output of phase shifter 3l is connected through double-pole double-throw switches 4l, l2 and 53 in tandem, with which are associated adjustable phase shift networks @14, 45 and 45, respectively, and short-circuiting straps 4l, 48 and 139, respectively, and a variable attenuator 50 to a second input of the phase sensitive detector Elia. The networks fit, l5 and it are adjustable to provide phase shift to the amounts of degrees, 90 degrees and 60 degrees, respectively. v

differential phase detector 20ct is relatively insensitive to the amplitude variations of the two voltage inputs thereto but highly sensitive to unequalities of the vectorial sum and difference of the two voltage inputs thereto. The phase indicator 2l displays phase shift on a direct-reading scale in the following manner: The equation for indicator 2i is where D is the deflection; A is a scale factor; F( I is the phase law of differential phase detector 29a. If a scale conforming to Equation l is affixed to indicator 2 l, such scale will indicate phase shift correctly if the scale factor A were adjusted to the proper value.

In the operation of Fig. 3 for the purpose of Calibrating phase shifter 23, the signal generator Ell is adjusted to the desired predetermined frequency, at which the phase shifter 23 is to be calibrated, such frequency being 3l kilocycles for the purpose of this explanation. Attenuators 33 and 50 are so adjusted as to permit differential phase detector 20a. to operate at its proper level of input voltages. As a first step, the phase shifter 23 is initially set at an arbitrary starting point` Then, with switches di, i2 and Q3 connected to their short-circuiting straps el', d8 and 49, respectively, the phase shifter 3'! is adjusted to provide a zero reading on indicator 2|. Now, switch 4| is connected to phase shifter 44 with the switches 42 and 33 remaining in their aforementioned positions, and the phase shifter 23 is adjusted until a zero reading is again established on indicator 2|. Switch 13| is again connected to the short-circuiting strap di, and phase shifter 3l is adjusted to establish a phase change in the same sense or direction as the next preceding phase change effected by phase shifter 23 until a Zero reading is re-established on the indicator 2|. Then switch ii is reconnected to phase shifter 44, and phase shifter 23 is adjusted in the same sense as in its next preceding adjustment until a zero reading is again established on indicator 2|. If the actual phase shift occurring upon the throwing of switch il from its short-circuiting strap 47 to the phase shifter 44 were exactly 180 degrees, then phase shifter 23 should have returned to the initial arbitrary starting point.

Ordinarily such exact occurrence will not be the case. Now, estimating the amount of phase shift by which the return to the initial arbitrary starting point in the calibration of phase shifter 23 was missed as indicated by line scale |48, Fig. 12, the phase shifter 44 is adjusted by about onehalf such estimated amount, and the foregoing procedure for Calibrating phase shifter 23 is repeated. In successive repetitions of the above Calibrating procedure, the amount of phase shift by which the initial arbitrary starting point on phase shifter 23 was missed will be progressively reduced until such amount is no longer discernible. Thus, an absolute 18o-degree phase standard is obtained; and a phase shift of 180 degrees from the initial arbitrary starting point on phase shifter 23 is established from the first step of the calibration. Thus, the phase shifter 23 now has the two calibrated points of 0 degree and 180 degrees.

As a second step in the calibration of phase shifter 23, the latter is returned to the initial arbitrary starting point, the switches di, G2 and i3 are connected to the short-circuiting straps 41, d8 and L19, respectively, and the phase shifter 31 is adjusted to provide zero reading on indicator 2 i. Next, switch 132 is connected to phase shifter A5, and the phase shifter 23 to be calibrated is adjusted in the same sense or direction as it was in the next previous adjustment thereof until the zero reading is returned to indicator 2 i. Then, switch 42 is reconnected to its short-circuit ing strap 48, and phase shifter 3l is adjusted until a zero reading is provided on indicator 2 Again switch d2 is connected to phase shifter 45, and phase shifter 23 is adjusted in the same sense or direction as its next previous adjustment until a zero reading is established on indicator 2|. lf the relative phase shift of phase shifter d were exactly 90 degrees, the phase shifter 23 would have returned to the 180-degree point established. via the first step of calibration. Ordinarily, this will not happen. Estimating the amount by which the return to the 180-degree point failed, the phase shifter (i5 is varied by about one-half such amount, and the calibration procedure according CII 8. to step 2 is repeated. From successive repetitions of this procedure, the amount of phase shift by which the 18o-degree point was missed will be progressively reduced until such amount is no longer noticeable. Thus, an absolute S30-degree phase standard is obtained. Following the basic procedural system described in above step l, the calibration of phase shifter 23 is established for every possible permutation such, for example, as 0i90; and l80i90- The SiO-degree and 270- degree points thus obtained are checked against each other with the absolute 18o-degree phase standard obtained in step l. Now the phase shifter 23 has the four calibrated points of 0 degree, degrees, 180 degrees and 270 degrees.

As a third step in the calibration of phase shifter 23, the basic procedure of step 1 is now utilized with switch i3 and the (S0-degree phase shifter 5 associated therewith to subdivide each of the four Sil-degree divisions, heretofore established on phase shifter 23, into three SO-degree divisions. Upon the completion of the third step of calibration, the phase shifter 23 will include twelve points of 30 degrees, Viz., 0 degree; 30 degrees; 60 degrees; 90 degrees; 120 degrees; 150 degrees; degrees; 210 degrees; 240 degrees; 270 degrees; 300 degrees; 330 degrees; and 360 degrees (or 0 degree).

Obviously a progression of submultiples of 30- degree phase Shifters, not shown, could be used to subdivide each Sil-degree section. However, it is convenient to utilize differential phase detector 20a and associated indicator 2| for such subdivision in the following manner. From Equation l,

D=AF() (2) Adjustment of the scale factor A provides an effective range from +5 degrees to -5 degrees, or a lil-degree phase shift standard, which, when properly subdivided on the scale of indicator 2| enables a further subdivision of each of the above 13D-degree sections of phase shifter 23.

As a fourth step in the calibration of phase Shifter 23, the latter is set at the 30-degree point or a multiple thereof previously established; and then phase shifter 31 is adjusted until a -5 degree reading is established on indicator 2 Next, phase shifter 23 is adjusted to provide a |5 degree reading on indicator 2 i. Now, phase shifter 3'! is adjusted to re-establish the -5 degree reading on indicator 2|. This procedure is repeated twice until phase shifter 23 has been adjusted through 30 degrees to the next adjacent point, which is a multiple of 30 degrees, as determined by indicator 2i. 1f the scale factor A were correct, the calibration should coincide exactly with the previously established Sii-degree subdivision or multiple thereof. If the calibration and the r:l0-degree subdivision do not agree, the scale factor A is adjusted, and the foregoing procedure repeated until agreement is obtained. Thus, the phase indicator 2| may be utilized to establish two lO-degree subdivisions between each two adjacent points which are multiples of 30 degrees. Upon the completion of the fourth step of calie bration, the phase shifter 23 includes the points 0 degree, 10 degrees, 20 degrees, 30 degrees, 40 degrees, 330 degrees, 340 degrees, 350 de grees, 360 degrees (or 0 degree).

As a fifth step in the calibration of phase shifter 23, the indicator 2| may be used in a manner similar to the fourth step to establish a 5-degree subdivision between each two adjacent l-degree subdivisions. In this case the phase shifters 36 and 31 are adjusted between the readings of degrees and 0 degree, or +5 degrees and 0 degree. At this state of calibration, the phase shifter 23 includes the points 0 degree, 5 degrees, degrees, l5 degrees, 20 degrees, 2 5 degrees 340 degrees, 345 degrees, 350 degrees, 3 55 degrees, 360 degrees (or 0 degree). As a sixth step in the calibration of phase shifter 23, the indicator 2| may be utilized in the manner of the fourth and fth steps to establish four l-degree Subdivisions between each two adjacent points that are multiples of 5 degrees. Upon the coinpletion of the sixth calibration step, therefore, phase shifter 23 will include the points 0 degree, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5degrees, 6 degrees, 7 degrees 354 degrees, 355 degrees, 356 degrees, 357 degrees, 358 degrees, 359 degrees and 360 degrees (or 0 degree). In a practical case, it was found that calibration to 9.1 degree for each multiple of 5 degrees was satisfactory.

Thus, the circuit of Fig. 3 shows the only components essential to the calibration system, viz., l, a phase shifter to be calibrated; 2, a plurality of phase shift networks adjustable to the multiples desired in the calibration; 3, an uncalbrated phase shifter continuously adjustable through 360 degrees; and 4, a detector including an indicator for identifying one arbitrary phase relationship, viz., quadrature, betweentwo alternating voltages as will be subsequently explained in connection with Fig. 7.

TRANSMISSION AND PHASE DETECTOR 2O IN FIG. l

The purpose of the diferential phase detector a shown in Figs. l, 2 and 3, respectively, is toI identify an arbitrary phase relationship, quadrature, between two alternating voltages and comprises a phase discriminator 24 that will now be described in connection with Figs. 4, 5 and 6. A characteristic of phase discriminator 25 is that it shall be substantially insensitive to amplitude Variations of the two input voltages whose relative phase is to be compared. The dinerential discriminator 24 shown in Fig. 4 is essentially a Wheatstone bridge comprising four arms of equal resistors, and having each of its two opposite diagonals connected to one of the two input voltages E1 and E2 whose relative phase is to be compared. The general phase relationship between the two input voltages E1 and E2 to the bridge in Fig. 4 is illustrated in Fig. 5.

Let it be assumed that then,

Ese( Een Ewen where n is any integer including zero ES=ED Thus, the amplitudes of the resultant voltages Es and ED are equal when the relative phase angle I is equal to degrees, 270 degrees, 450 degrees, etc.; and are independent o f the amplitudes of E1 and E2 as shown in Fig. 6,

The bridge phase discriminator 24 in Fig. i may be used as a deflection bridge wherein the resultant voltages Es and ED may be utilized to measure the relative phase thereof.

From Equations Zhand 3,

Equation 6 represents the phase law governing the utilization of the bridge phase vdiscriminater 24. From Equations 4, 5 and 6` I Cotal (ogrnolz-cos (o-tm) Substituting Equation 8 in Equation 7, then E2 cos 1) cos 1 2 f Hence,

20 log ZI-2:

I 12r+mr between two input voltages E1 and E; will be distinguished regardless of the amplitudes of those voltages;

2. For relatively small departures from 7|' @--iW/r the relatively small amplitude differences between the input voltages E1 and E2 will cause relatively insignificant errors;

3. Although equality of the resultant voltages nominally exists every 180 degrees, this is only true when the four resistance arms in Fig. 4 are exactly equal. If those arms are unequal, balance will exist for all where A6 is the departure angle from 90 degrees between input voltages E1 and E2; and

4. Either of the balance conditions mentioned in next preceding item 3 may be used to identify phase shifts in multiples of 2r exactly, regardless of imperfections in the phase discriminator. This meets the basic prerequisite for the absolute method of phase subdivision described above under Calibration of Phase Shifter.

The bridge phase discriminator 2 according to Figs. 4, and 6 is incorporated in the differential phase detector 250; shown in Figs. 7 and 8 which will now be explained. In Fig. 7, E1 and E2 represent two alternating voltages whose relative phase is to be measured. Voltage E1 is applied through preamplifier 55 to the vertical diagonal of bridge phase discriminator 2A; and voltage E2 is applied through preamplifier 5e and transformer'l to the horizontal diagonal of bridge phase discriminator 2li. Output voltages Es and En from the bridge phase discriminator 24 represent the vectorial sum and difference of voltages E1 and E2, at the bridge phase discriminator 2li, and are applied through buffer amplifiers 58 and 59 of identical gain respectively, to a differential rectification system comprising solid rectiers or unidirectional devices t8, BI, 62, and |53; capacitors 64, 65, 68 and 61; and resistors 68, BQ, lil, 3|, 12, 73, 1li and '55.

When the output voltages Es and En from the phase discriminator 2d in Fig. 7 are equal, and assuming resistors '52, i3, i4 and 'I5 are equal, then no direct current potential will occur across the points A-B and C-D in the output of the differential rectifier system. When the output voltages E1 and E2 from the phase discriminator 24 are unequal, then direct current potentials will occur in output of the differential rectifier system across the points A--B and C-D in equal magnitudes but opposite phase, the latter being due to the poling of rectiiiers 6|), 5|, E2 and 53.

' The magnitudes of the rectified voltages are proportional to the difference in magnitude between the voltages Es and ED. The rectied voltages are applied to the control grids of amplifier tubes 16 and l? included in a differential direct current amplifier 'i8 comprising, in addition, a cathode feedback including xed resistors 82, 83 and Ed and adjustable resistors 85 and B8; plate resistors 81 and 83, and a center-zero milliameter serving as the previously-identified phase indicator 2i. Resistors 89 and Si] provide biasing potentials for the control grids of amplifier tubes 'f6 and V. Capacitor 9| provides a high frequency by-pass to ground at points C and A.

Adjustable resistor 8G is adjusted to establish the same flow of direct current through plate resistors 8i and 88 whereupon no current will flow through phase indicator 2| when the latter resistors are equal. Adjustable resistor 85 controls the amount of negative cathode feedback and thereby the gain of amplifiers 1B and '|f in equal amounts provided resistors B2 and S have the same amount of effective resistance.

Now, let it be assumed that buffer amplifiers 58 and b9 have the same gain. When the voltages Es and ED are unequal, rectified direct current voltages will appear across the points A-B and C-D to cause an unbalance in the amount of space current iiowing in the direct current amplifiers 'l5 and Ti. This will result in a flow of current in the phase indicator 2i whereby a corresponding deflection is produced on the scale thereof. f the scale were proportioned according to the scale law given in above Equation l, and the factor of proportionality A were adjusted properly by varying resistor 85, the phase indicater 2| would show directly the change in phase from the quadrature relation of voltages E1 and E2 at the input of the bridge phase discriminator 2d. Thus, the differential phase detector 20a in Fig. 7 provides an arrangement for identifying a definite phase relationship, via., quadrature at the input of phase discriminator 2li, between two alternating input voltages, and to indicate directly on suitably calibrated indicator 2| any change in such relationship.

Fig. 8 is essentially the same as Fig. 7 except the former includes certain features to be identined hereinafter for facilitating the use of Fig. 7.

Switches |31 and |538 serve to interrupt the signal voltages applied to the control grids of amplifier tubes 'iE and Ti thereby permitting adjustment of resistor 36 to establish equal space currents through the amplifier tubes it and as previously mentioned. Voltage dividers 99 and lili? and ii@ and |02 serve to vary the sensitivity i of dierential amplifiers 'I6 and 'il via single-pole double-throw switches W3 and IM, respectively, whereby use of the fine and coarse scales of indicator 2| is permitted. For maximum sensitivity, the resistance of voltage dividers 99 and Slis and H0 and H62 must be high. Very small amounts of grid current in the amplifiers 'i5 and 'il' flowing through the resistors of the respective grid circuits will develop corresponding grid biases. The amount of grid current nowing in the respective grid circuits maybe the same or different depending on the characteristics of the respective amplifier tubes i6 and il. To preclude change in bias and thereby the production of a false reading on the scale of phase indicator 2| when actuating switches w3 and IM from the fine to the coarse scale of phase indicator 2|, and vice versa, the resistance of the grid circuits inust be kept constant. This is achieved by makingV the resistances of resistors i and |0| equal to that of resistors 99 and m2. As the voltage dividers 99 and H39 and |02 and H0 complete the direct current path from the points B and D to ground i the resistors et, 59, 'it and 1| may be omitted in Fig. 8, if desired. Single-pole singlethrow switch |05 enables a varistor I to be connected in shunt of indicator 2| whenever desired. rihe varistor H15 possesses high initial resistance for applied voltage of relatively low magnitude and decreasing resistance for applied voltages of increasing magnitude. rlhus, the varistor |65 will be substantially ineffective as a shunt for relatively small deflections on the scale of phase indicator 2| but an effective shunt for larger deflections thereby permitting maximum sensitivity of phase indicator 2| for small departures from the quadrature relationship of input voltages E1 and E2, and low sensitivity of phase indicator ZI for large departures from such relationship. In the ideal case, it happens that El E2, and ES ED w/ when the quadrature relation occurs between the input voltages El and E2. For the purpose of checking the circuit of Fig. 8, a singlepole singlethrow switch @t is closed, and single-pole doublethrow switches 95 and 95 are moved to their contacts H and F, respectively. Via voltage dividers `9i and 98, a voltage E1 d is applied to the inputs of buffer amplifiers 58 and 59 and the gain thereof is adjusted until no voltage appears across the .points A-B and C-D as shown by a zero deflection on indicator 2 I.

Referring again to Fig. 8, buffer amplifiers 55 and 56 are connected through buffer amplifiers H5 and IIi, respectively, to the input of a differential transmission rectification system 29h which is identical with the differential phase rectication system hereinbefore described as connected to the output of ampliers 53 'and 59 and which has its output connected to an indicator 25 similar to the phase indicator 2| but calibrated in suitable transmission units. The differential transmission detector 2Gb and indicator 25 serve to measure the difference between the amplitudes of input voltages E1 and E2. Singlepole double-throw switch IIS enables the application of equal voltage inputs to buffer amplifiers IE5 and IIB. Buffer amplifier I2ii provides a monitoring voltage which may be supplied to a meter, not shown, or which may be utilized for an automatic voltage control AVC in Fig. 1 as herein explained.

Fig. 9 shows in further detail the varistor It in Fig. 8 for the purpose of providing each of indicators 2! and 25 with a desired non-linear scale. Due to manufacturing variations of individual rectiers, the individual rectiers IIA, as shown in Fig. 9, may be connected directly to leads I2I and |22 via a strap I23, or via individual trimmer resistors I 24, or several rectiers lill may be connected through one trimmer resistor I25. The resistors IE6 and H25 serve to adjust the characteristics of individual varistors IIA.

OPERATION oF FIG. 1

(a) For measuring Zoss or gain In the operation of Fig. 1, the apparatus under test I0 is tested with the testing waves f which vary from 50 kilocycles to 3600 kilocycles, and which are supplied to one input of each of modulators i6 and Il. The other input of each of these modulators is supplied at the same time with the waves f4 varying from 8l kilocycles to 3631 kilocycles. Hence, the outputs of modulators I6 and I'I include a component having a frequency of 31 kilocycles at which the phase and transmission characteristics of the unknown and standard paths are compared in the differential detector 2l) and measured by the indicators 2| and 25 connected thereto.

The attenuators I2 and 22 are individually adjustable to one of the other of the two positions O-decibel loss and 4D-decibel loss. These attenuators are connected together by the mechanical connection 32 in Figs. 1 and ll so that when attenuator I2 is in the O-decibel loss position, the attenuator 22 is in the 4D-decibel loss position; and so that when attenuator I2 14 is in the 4G-decibel loss position, the attenuator 22 is in the 0-decibel loss position.

For effecting a null operation of Fig. 1, zero center scales are provided on indicators 2I and 25; a 4U-decibel input level is to be supplied to the dierential detector 20 from each of the unknown and standard branches; the insertion phase shift of the apparatus under test i0 is read directly from the calibrated phase shifter 23; and the loss or gain of the apparatus under test il) is read directly from calibrated attenuator I9.

The standard branch comprises as xed transmission conditions (l) a 2li-decibel gain in modulator Il, (2) a 2li-decibel loss in phase shifter 23, and (3) a li0-decibel loss for either gain or loss measurements by means of (a.) the e0-decibel loss position effective in attenuator 22 when the 0-decibel loss position isY effective in attenuator i2 for loss measurements, or (b) the 40- decibel loss position in attenuator I2 when the 0-decibel loss position is eifective in attenuator 22 for gain measurements. The unknown branch comprises as fixed transmission conditions (l) a +20-decibel gain in modulator It; (2) a Il-decibel loss or 4D-decibel loss due to position of attenuator I2 when effecting loss or gain measurements, respectively; (3) gain or loss" of the apparatus under test I9; and (4) effective attenuation of calibrated attenuator IS. As a consequence, an effective ISG-decibel loss should be present in each of the unknown or standard branches whereby the xed conditions in each branch should add algebraically at all times to the two 4D-decibel input levels to differential detector 2U.

Assuming for the moment that (a) apparatus under test I9 is removed from the circuit of Fig. l and replaced with a suitable short-circuiting strap say, for example, a section of coaxial cable; (b) attenuator I2 is on the O-decibel loss point; and (c) attenuator 22 of the standard branch is on the 4U-decibel loss, then calibrated attenuator I@ in the unknown branch will actually include a G0-decibel loss to compensate for the +20-deci bel gain of modulator I6 whereby a u d0-decibel signal is applied to the unknown input of the differential detector 20. At the same time the standard branch will include +20-decibel gain in modulator I'I, 20-decibel loss in phase shifter 23, and 4U-decibel loss in attenuator 22 whereby a db-decibel signal, due to the attenuation of attenuator 22 alone, is applied to the standard input of the differential detector 2li. As a consequence, each of the three dials on attenuator I9 will read zero as will be hereinafter explained.

Assuming further for the moment that (a) the apparatus under test I Il is still removed from the circuit of Fig. 1 and the short-circuiting strap of coaxial cable is still substituted therefor (b) attenuator I2 of the unknown branch is on its 40- decibel loss point, and (c) the attenuator 22 of the standard branch is on its "O-decibel loss point, then calibrated attenuator I9 of the unknown branch will actually include 20-decibel loss (4U-decibel loss in the attenuator I2, +20- decibel gain in modulator I and 2li-decibel loss in attenuator I9) to compensate for +20-clecibel gain in modulator It whereby a -40decibel" signal is applied to the unknown input of differential detector 29. At the same time, a +20- decibel gain in modulator II and a 20-decibel loss in phase shifter 23 enable a 40-decibel signal due to the e0-decibel loss in attenuator I2 to be applied to the standard input of differential detector 20. As a consequence the three .dials of attenuator i9 will read 40`decibel'lossffas will be hereinafter explained. Y

The three dials on attenuator vl 9 are adapted to show readings of gain or loss directly thereon as shown in Fig. 10 in the following manner. These dials are so arranged rwith twoconcentric sets of calibration numeralsadjacent the periphery thereof that the outer setof numerals reads gain directly and the innerfsetl reads loss directly. The outer calibration on each dial is stamped in red numerals adapted to be illuminated for indicating gain as will be hereinaiter explained, in the forward direction as the dials are rotated in a counter-clockwise direction while the inner calibration is stamped in white numerals for indicating loss, as will be subsequently described, in the reverse direction as the dials are rotated in a clockwise direction; In this connection, it will be noted that the outer calibration of the l-decibel dial includes-'both red and white numerals as will be pointed outhereinafter. Thus, the three dials are-arranged for so-called reverse reading of loss v These calibration numerals will be illuminated for indicating gain or loss measurements as hereinafter explained in connection with-Fig. 1l. `It will be noticed in Fig. 10 that the l0-decibel step isV omitted from the 1.0-decibel dial. The attenuator I9 and its three dials are calibrated as follows:

l db dial (Actual db loss in attenuator 19) 0 l0 2U 30 40 50 Thus, attenuator i9 will read as almaximum.

Gain:

30.0 db on the 10 db dial 9.0 db on the 1.0 db dial" 1.0 db on the 0.1 db dial 40.0 db total v Loss:

50.0 db On the 10 db dial i 9.0 db 011 the 1.0 db dial 1.0 db 0n the 0.1 db dial 60.0 db total The foregoing may beA illustrated as `follows:V

Actual loss in attenuator 19; A being the vreading of this attenuator Light Indica- Setting o Attenuator l2 l tion (604A) ab. (20-l-A) db. (2c-A) db.

'An illuminating system shown in Fig. 11 is associated by electrical connection 33"'in Fig; 1 with the three-dial attenuator 9 and'attenuator `l2. Fig'. 1, to indicate'by a'red 'pilot lamp"200\ that gain is being measured or byfa white pilot lamp 20! that -loss'is being measured'.` In addition, a white lamp 280:1 and a white lamp 20m are positioned immediately in back of the red and white color designations of the numerical calibrations of the three dials of attenuator I9, Figs.l0 and 11 as hereinbefore mentioned. Accordingly, in a manner to be explained subsequently herein, the red pilot lamp 200 and corresponding red numerical calibrations on the dials of attenuator I9 visually indicate that gain is being measured; and the white pilot lamp 201 and corresponding white numerical designations on the dials of attenuator I9 visually indicate that lossis being measured. The circuit, as shown in Fig. 1l, is arranged to indicate visually that gain measurements are being effected. v

Referring now to Fig. ll, panel 202 associated with attenuator i2 includes a manually operated switch 203 for adjusting attenuators l2 and 22 via mechanical connection 32 to the O-decibel loss and LiO-decibel gain positions, respectively, for measuring loss or to the 40-decibel gain and il-decib-el'loss positions, respectively, for measuring gain. The switch 203 also connects via electrical connection 33, battery 204 to the lamps 20E, 201, zitta` and 20m, depending on whether loss or gain is being measured. A microswitch 201, i. e., a single-pole double-throw switch actuated by a cam not shown, attached to the shaft of the -10decibel dial, Fig. 11, of attenuator I9 effects a change of lamps as the measurements go from gain to loss, or vice versa, when the 20-decibel changeover point is passed in a manner that will presently appear. In achieving such measurements there is a point at which the measurements change from gain to loss and vice versa, whereby the lamps are caused to change from red 200 to white 20| and vice versa. The changeover point isdetermined by the 4Q-decibel loss in attenuator I2 in the following manner.

Initially, the circuit constants of the unknown branch of Fig. 1 are preselected so that at all times, as above explained,

lLO-decibel loss in gain or loss attenuator 12 in -iin apparatus the unknown under test l0 branch and of the standard branch of Fig. 1 are preselected so that at all times as above shown:

4o-dec1be110ss'm attenuator 12 in -iloss of phase =l attenuation 60 db loss attenuator 19 60 db loss the standard shifter 23 branch l-decibel loss in l-decibelloss 19 decibels in attenuator 12 in apparatus attenuator 19 4O-decibel loss in attenuator l2 60 db loss to at s Hence, the'apparatus under test 10 has a 1- 17 decibel loss since l-decibel attenuation must be fremoved from attenuator I9; and the white lamps 20| and 20 Ia are illuminated in Fig. 11.

EXAMPLE NO. 3

iO-decibel loss in S-decibcl loss 12 decibels in attenuator 12 -iin appratus -lattenuator 19 l 60 db loss Thus, the apparatus Linder test 10 has an 8- decibel loss since B-decibel attenuations must be removed from attenuator I9; and the white lamps 20| and 20| a are illuminated in Fig. 11.

EXAMPLE NO. 4

li-decibel loss in l-decibcl gain 2l decibels in attenuator l2 -I- inapyliratus attenuator 19 li0-decibel loss iu -decibel gain 28 decibels in attenuator 12 -lin aplliratus attenuator 19 60 db loss rlhus, the apparatus under test l0 has +8- decibel gain since 8-decibel attenuation must be added to attenuator I9; and the red lamp 200 and white lamps 200a are illuminated in Fig. 11.

Therefore, the L10-decibel attenuator I2 insertable in the unknown branch in Fig. 1, in View of the over-all 4C-decibel gain and 60- decibel loss measurable as previously pointed out, xes the changeover in readings from loss to gain and vice versa at the decibels normally in attenuator I 9 whereat the apparatus under test l0 has neither loss nor gain as hereinbefore mentioned. Hence, when attenuator IS includes actual attenuation of a value less than "20 decibels, the apparatus under test 10 has loss and the White lamps 20| and 20Ia are always illuminated in Fig. 11; and when attenuator i9 includes actual attenuation of a value above 20 decibels the apparatus under test l0 has gain and the red lamp 200 and white lamps 200:1 are illuminated. Thus, the lamps in Fig. 11 change from red to white lamps, and vice versa, depending on whether attenuator |9 has more or less than 20 decibels of actual attenuation. The microswitch 20'] in Fig. l1 mounted on the shaft of the l-decibel dial, Figs. 10 and 11, serves to effect the lamp changing mentioned hereinbefore.

Also, the three-dial attenuator I 0 in Fig. 1 may be used to measure loss as follows:

Actual loss in attentor 19 Readings on 3 dials of at- Setting of attenuator 12 tenuator 19 Loss in Apparatus 10 .0 db. .0 db loss. 6 db loss.

Jamo 0:00

Actual loss Readings on in attenua- 3 dials of attor 19 tenuator 19 Setting of attenua- Gain in Aptor 12 paratus 10 40.0 db gain. 20.0 db loss. 10.0 db loss. +200 db gain.

To effect loss or gain measurements in Fig. 1, the attenuators I2 and 22 are initially adjusted to the G-decibel loss or L10-decibel loss positions, respectively, for loss measurements, or to the -decibel gain or ll-decibel positions, respectively, for gain measurements, with the apparatus under test 10 connected in circuit. The differential transmission detector lilla compares the signal outputs from the unknown and standard branches, and shows any dierence therebetween on transmission indicator 25. For null measurements, attenuator I9 is adjusted to balance the unknown and standard signals until a zero-center reading is produced on indicator 25. The loss or gain of the apparatus under test 10 is read directly from the three dials on attenuator I9 in the manner previously explained. For apparatus under test 10 having gain, the actual attenuation of attenuator i0 increases as the gain of each apparatus increases so as to measure gain, and for apparatus under test i0 having loss, the actual attenuation of attenuator i9 decreases as the loss of such apparatus increases so as to measure loss.

Example No. 1.-Assume 0decibel gain or loss for apparatus under test 10, then (a) The attenuator I2 is adjusted to the 40- decibel loss position while the attenuator 22 is adjusted to the O-decibel loss position;

(b) The white lamps 20| and Zilla are illuminated;

(c) The attenuator I9 reads 0-0-0 on its 10- deciloel, 1.0 decibel and 0.1 decibel dials, respectively, i. e., neither gain nor loss;

(d) The actual attenuation in attenuator l0 is 20 decibels so as to nullify the |20decibel gain of modulator I0;

(e) The unknown branch has a0-decibel loss comprising zlll-decibel loss of attenuator I2, +20- decibel gain of modulator I0, and 20-decibel loss of attenuator i0; and

(1") The standard branch has a l0-decibel loss comprising lll-decibel loss of attenuator I2 in unknown branch, -f-ZO-decibel gain of modulator Il, Ztl-decibel loss of phase shifter 23.

Therefore, the apparatus under test l0 has neither loss nor gain.

Example No. 2 Assume +14.6decibel gain for apparatus under test 10, then (a) The attenuator I2 is adjusted to the 40- decibel loss positions while the attenuator 22 is adjusted to the O-decibel gain position;

(b) The red pilot lamp 200 and associated white lamps 200er are illuminated;

(c) The attenuator i9 reads la-decibel gain 1 on the 10-decibel dial, 4 on the 1 0-decibel dial, and 0.6 decibel on the Ill-decibel dial);

(d) The actual attenuation in attenuator I9 is 34.6 decibels comprising 30.0 decibels per the lil-decibel dial, Ll-decibels per the 1.0-decibel dial and 0.6 decibel per the (Ll-decibel dial;

(e) The unknown branch has Llli-decibel loss comprising LlO-decibel loss of attenuator I2, -{1f1.6 decibel gain of apparatus under test 10, +20- decibel gain of modulator 115, and S-decibel loss of attenuator I 9; and

(f) The standard branch has eil-decibel loss as identied in paragraph (f) of Example No. 1.l

Thus, the apparatus under test 10 is an active network.

Example No. .3f-Assume lil-decibel loss in the apparatus under test 10, then (a) The attenuator |2 is on the O-decibel loss position and the attenuator E2 is on the 40-decibel loss position;

(b) The white lamps 20I and 29m are illuminated;

(c) The attenuator I9 reads 11i-decibel loss (1 on the 10-decibel dial or 9 on the 1.0-decibel dial, -{-l.0 on the 0.1-decibel dial) (d) The actual loss in the attenuator I9 is 50 decibels comprising 40 decibels on the l-decibel dial, 9 on the 1.0-decibel dial and 1.0 decibel on the 0.1 decibel dial;

(e) The unknown branch has LiO-decibel loss comprising li-decibel gain of the attenuator I2, l-decibel loss in the apparatus under test 10, -I-20-decibel gain of modulater I6, and `LED-decibel loss of attenuator I9; and

f) The standard branch has iO-decibel loss comprising 40-decibel loss of attenuator 22 in the unknown branch, -l-ZG-decibel gain of modulator 17, and 20-decibel gain in phase shifter 23.

Example No. 4.-Assume 54.9-decibel loss in the apparatus under test 10, then (a) The attenuator I2 is on the O-decibel loss position and attenuator 22 is on the 4G-decibel loss position;

(h) The white lamps BSI and 29m are illuminated;

(c) The attenuator I9 reads 54.9-decibel loss as follows: 5G decibels on the 1.0-decibel dial, 4 decibels on the 0.1-decibel dial and 0.9 decibel on the 0.1-decibel dial;

(d) The actual loss in the attenuator is 5.1 decibels comprising decibel on the 10-decibel dial, decibels on the 1.0-decibel dial and 0.1 decibel on the 0.1-decibel dial;

(e) The unknown branch has Llli-decibel loss comprising O-decibel loss in attenuator I2, 54.9- decibel loss in the apparatus under test 10, +20- decibei gain in modulator I5, and 5.1-decibel loss in attenuator I9; and

The standard branch has the ifi-decibel loss as identified above under item (f) of Example No. 3.

In connection with the 1.0-decibel and 0.1- decibel dials of attenuator I9 shown in Fig. 10, it will be understood that each dial may include eleven steps in which event the maximum reading oi' the 1.0 and 0.1-decibel dials would be 10.0 decibels and 1.0 decibel, respectively, whereupon the maximum reading of the attenuator I9 would be 61.0 decibels; or that each of the 1.0-decibel and 0.1-decibel dials may include ten steps in which event the maximum reading of the 1.0- decibel and (L1-decibel dials would be 9.0 decibels and 0.9 decibel, respectively, whereupon the maximum reading of the attenuator I9 would be 59.9 decibels. As a consequence, the circuit constants of Equations 10 and 11 would be 61.0 decibels for the SLG-decibel attenuator and 59.9 decibels for the 59.9-decibel attenuator just mentioned.

(b) For measuring phase shift To achieve phase measurements in Fig. 1 with the apparatus under test 10V connected in circuit, the phase dierence may be read directly on phase indicator 2 I. For null measurements the calibrated phase shifter 23 is adjusted to provide a zero-center reading on phase indicator ZI, and the amount of phase shift inserted by the apparatus under test 10 is read directly from the calibrated phase shifter 23. For these measurements, the first measuring step is to establish a Zero for the over-all phase measuring system in Fig. 1 in the following manner. This is done by substituting a section of coaxial cable for the apparatus under test 10 in Fig. 1, or otherwise effectively substituting a short circuit for it, to

achieve a null measurement. Next, phase shifter 23 is adjusted to establish a Zero-center reading on phase indicator 2I. Now the coarse and fine reading scales I3'I and ME, respectively, associated with the phase shifter 23 in Fig. 12 are slipped via their respective friction clutches as described hereinafter in connection with Figs. 12, 13 and 14, until both scales show a Zero reading. This constitutes the over-all zero for the phase measuring system of Fig. 1. As a second measuring step, the coaxial line section is replaced with the apparatus under test 10, and using the null procedure, the zero reading is reestablished on phase indicator 2| by appropriate adjustment of phase shifter 23. The amount of phase shift introduced by the apparatus under test 10 is read directly from the coarse and fine scales |31 and HI8, respectively, of the phase shifter 23 in Fig. 12. This obviates the need for adding or subtracting readings as would be required with scales incapable of the slippage above mentioned.

Modulators I5 and Il are constructed such as to have identical gain-versus-frequency characteristics. They are also constructed such that a linear relationship exists between changes in the amplitude of the input waves of frequencies f and f4 and the amplitude of the output waves of the difference frequency Jil-f, or 31 kilocycles. Hence, by varying the amplitude of waves of frequency f4 the outputs of modulators I5 and Il may be changed by the same factor. The AVC connection shown in Figs. 1 and 8 maintains substantially constant the amplitude of the standard input into the differential detector 2li, thus compensating the gain-frequency characteristics of modulator I5 and modulators I3 and I1, and changes in amplitude of waves of f at the splitting pad I3 because of possible reflections from apparatus under test arising from impedance mismatch. This permits direct reading of the scales of the phase and transmission indicators 2| and 25, respectively, as herein described.

OPTICAL CANI FOR CALIBRATED PHASE SHIFTER 23 IN FIG. 1

As the calibration of phase shifter 23 includes residual error at different points, its calibrated scale would be non-linear if the deviations due to such error were incorporated therein. This would mean that the calibrated scale would be fixed relative to an arbitrary point on the shaft on which the latter is coaxially mounted, and once the scale is moved relative to such arbitrary point, the utility of the calibrated scale would be lost. In the calibration of phase shifter 23 as explained hereinbefore, its calibrated scale is made linear and is associated with a movable index representing the amount of its deviation from linearity due to residual error, in the manner that will now be described.

Referring to Fig. 12, phase shifter 23 comprising the well-known four-quadrant sine condenser having a rotor and stators is affixed to one end of shaft I3 on which is mounted spider I3I, gear |32 and a transparent drum |33. Spider I3| shows in Fig. 13 and arm |34 attached rigidly at its center to the shaft I30 and adapted with a pair of shoes |35 on its opposite ends held in frictionai engagement with the inner periphery of a drum I36 carrying on its outer periphery a scale I3'I which contains thirty-six steps of 10 degrees each in linear form. The scale I3? provides a coarse reading of the effective phase shift of phase shifter 23 at a given instant. A compression spring I38 regulates the friction between the 21 shoes |35, |35 and inner periphery of drum |36. Ring gear |39 engages spur gear |40 which can be actuated by a finger knob |4|. Thus, rotation of knob |4| serves to rotate the coarse scale |31. A fixed index |4|a is associated with coarse scale |31.

A worm gear |42 meshing with gear |32 is driven by a shaft |43 mounted in spur gear |54. The latter engages a further spur gear |45 connected rigidly to one end of a shaft |43 whose opposite end is terminated in a finger knob |41 which has nne scale |48 frictionally secured thereto and adapted with ten 1-degree divisions in a linear manner, representing one of the thirty-six IO-degree steps on coarse scale |31 as above mentioned. This frictional engagement is illustrated in Fig. 14 in which a friction plate |49 anchoring adjacent ends of spaced rods |58 and whose opposite ends are fixedly mounted in a button |52. A compression spring |53 engaging an inner surface of button |52 serves to con trol the frictional engagement between the ne scale |48 and plate |48. By pushing button |52 in a right-hand direction, the pressure of friction plate 4S on fine scale |48 is released whereby the iattei` may be rotated relative to shaft |53 for a purpose that will later appear; and at the same time friction plate |49 engages the panel latta thereby locking the shaft |46 against rotation.

Thus, as shown in Fig. 12, the phase shifter 23, a coarse scale |31 and a line scale |48 are associated with the shaft 3|). The gear ratio between the coarse and fine scales |31 and |48 is l to 36 whereby the fine scale |48 is caused to make thirty-six complete revolutions for each complete revolution of the coarse scale |31; or, in another' aspect, the ne scale |38 makes one complete revolution for each -degree step of the coarse scale |31.

When the phase shifter 23 was calibrated in the manner hereinbefore explained, it Was found that the O-degree or 10-degree point of the line scale |33 did not always fall on the same ixed index for each point calibrated. It so happened that the li-degree or lil-degree point of the fine scale |58 tended to fall to the right or left of the fixed index as well as occasionally thereon. This meant that the phase shifter 23 had residual error which was peculiar to its adjustment at a given instant. rlhus, the phase shifter 23 had a different index for different adjustments thereof whereby erroneous readings tended to result. As a consequence it was found necessary to provide a movable index for the fine scale |48 in order to com- L pensate for the residual error of the phase shifter 23 in a manner that will now be described.

Referring again to Fig. 12, a negative photographic film |55 containing a calibration curve |51 of phase shifter 23 is afxed to the periphery of transparent drum |33 in a manner that will be mentioned later. A beam of light from light source |53 is focussed by lens |59 on a mirror |68 and projected thereby through the calibration curve |51. As a consequence, a relatively short and narrow strip of illumination |53 is projected onto frosted surface |6| of a transparent window |32 which is positioned in proximity to ne scale |48. Due to the opacity of film |53 at all points but the calibration curve, it is obvious that the light beam will pass only through the calibration curve |51 thereon. As the frosted surface |51 lies in the same vertical plane with that of the ne scale |48, no parallax exists between the latter and the projected portion of 22 the calibration curve |51. It will be noticed that the beam of light emerges from the left-hand side of lens |59 in approximately parallel lines which is convenient but not necessary. It is only required that the divergence or convergence of the beam of light be unaltered after the calibration curve |51 is fixed on the transparent drum |33. Thus, the projected light beam |53 on the frosted surface |3| of the transparent window |82 provides a shifting Zero which represents the residual error of phase shifter 23 for diiferent adjustments thereof and which constitutes a moving index against which the fine scale |48 is read for achieving the measurement of insertion phase of apparatus under test 10 as hereinbefore described in. connection with Fig. 1.

1n making correction curve |51 in Fig. 12, a strip of transparent nlm or the like, not shown, is initially applied to the periphery of transparent drum |33, in the stead of lm |56. As a starting point, the coarse and fine scales |31 and |28, respectively, are set at their zero points; and a suitable mark is made on the nlm and transparent drum |33 to indicate a point A common to each thereof, Fig. 12, for a purpose that will become evident subsequently. Thereafter, the phase shifter 23 is calibrated in the manner hereinbefore explained. In this calibration it is neces sary to adjust the phase shifter 23 at each point calibrated so that the 0-degree or 5-degree or other scale division on fine scale |48 is referred to an index. Since the latter shifted or moved with reference to a fixed index for different calibrated points of phase shifter 23 as previously pointed out, an ink dot is now placed on the 111m in such a way that its projection onto the screen |6| is adjacent to the desired subdivision of the ne scale |48.

Thus, there will be an ink dot on the nlm for each calibrated point of phase shifter 23 to repn resent where the 0-degree or lll-degree point fell; and for the purpose of the present explanation, it is` assumed that there will be an ink spot on the film, for example, at each of the thirty-six lil-degree steps calibrated on the coarse scale |31. Since the coarse scale |31, transparent drum |33 and fine scale |48 are actuated by the common shaft |30 as shown in Fig. 12, it will be evident that the 'transparent drum |33 makes one revolution for each revolution of the coarse scale |31 whereas the iine scale |48 made thirty-six revolutions for each revolution of the coarse scale |31 as previously mentioned.

Therefore, there will be at least thirty-six ink dots spaced along the length of the film on the transparent drum |33. Next, the lm is removed from the transparent drum |33, then laid on a flat surface, and the ink dots thereon connected together by a reasonably heavy ink line with the aid of a so-called French curve. The connected ink dots will constitute a curve which is approximately sinusoidal for each of the four quadrants through which the rotor passes in calibrating the phase shifter 23 through 363 degrees. As

- a consequence, the over-all inked curve on the film will comprise essentially four sinusoidal curves connected together, each of the latter curves having a hill" and a valley portion.

Finally, the over-all inked curve on the film is photographed and the negative thereof conn stitutes the film |56 on the transparent drum |33 while the four connectedsinusoidal curves form the correction curve |51 on the nlm |53 in Fig. 12. The negative film |56 is applied to the periphery of transparent drum |33 so that the starting point Ai on the negative nlm |56 is disposed precisely at the starting point A on the transparent drum |33, Fig. 12. Thus, a relatively short and narrow strip of illumination |63 from the lamp |58 will pass through the correction curve |51, but will be blocked by the negative portion of the film |56 adjacent thereto. This strip of illumination |63 is projected onto the frosted surface |61 of the transparent window |62 and moves back and forth therealong to constitute the movable index for the fine scale |28 as above explained.

An important aspect of the foregoing optical correction curve is that both the coarse and fine scales |31 and |48, respectively, may be adjusted to any desired relation with respect to the rotor of phase shifter 23 and the correction curve |51 on the transparent drum |33. In this connection, it will be noted that the rotor of phase shifter 23 and the transparent drum |33 are rigidly positioned in fixed relation on the shaft |30 but the coarse scale |31 is rotatable relative to shaft |32 via spider |3| and the ne scale |23 is rotatable relative to shaft |36 via finger knob |31 and the plate |119 frictionally engaging the latter as pointed out previously. Hence, the coarse and fine scales |31 and |48, respectively, may be adjusted to any desired relative positions without disturbing the fixed relation between the correction curve |51 and the rotors of the phase shifter 23 in Fig. l2. The latter ensures that the proper amount of correction is made at the same adjusted positions of the rotor of the phase shifter 23 irrespective of the actual reading of the coarse and fine scales |31 and |48, respectively. Hence, it will be noted that the correction for residual error is associated with the rotor and not with the coarse and/ or fine scales per se. Accordingly, the relative movement between the coarse and fine scales performs essentially an algebraic addition or subtraction depending on whether they are moved in a forward or backward direction. In other words, once the correction for the residual error of phase shifter 23 is established and maintained at all times, it is immaterial what arbitrary reading is given to 'the coarse and fine scales |31 and |113, respectively, at a given instant.

Fig. 15 shows a modification of Fig. 12 in that the former provides a correction curve |12 which is longer than but similar to the correction curve |51 in Fig. 12 and which may be useful in certain cases. Thus, Fig. 15 provides a correction curve |12 which may be contained on reels in the manner of nlm and which may be any length Whereas the correction curve |51 according to Fig. 12 is fixed in length by the periphery of transparent drum |33. Referring to Fig. 15, engaging spur gears |66 and |61 are formed with a predetermined ratio and are substituted for the transparent drum |33 in Fig. 12. A shaft |68 has one end mounted in the spur gear |61 and its opposite end rigidly attached to transparent drum |69 which is similar to the transparent drum 533 in Fig. l2. Two sets of sprocket teeth |13 of the usual structure are disposed on the periphery of transparent drum 63 adjacent the opposite ends thereof, and are adapted to accommodate the two spaced sets of familiar apertures formed adjacent longitudinal edges of a film 11| carrying a correction curve |12 for the phase shifter 23. The remaining elements in Fig. l5 are identical with corresponding elements in Fig. 12. The film |1| in Fig. 15 may `be of the endless type or 2e stored in magazines as disclosed in the Slonczewski patent, supra.

Fig. 16 is an alternate embodiment of the arrangement of Fig. 12 in that ithe former includes a linear movement for effecting phase shiftr whereby the correction curve for residual error is caused to mov-e in a linear direction. The arrangement of Fig. 15 may be substituted in Fig. l between the lines X-X and Y-Y. Referring to Fig. 16, a phase shifter 206 comprises a pair of conductors 20|, 26|, a slider pick-up 232, a termination 233, and an amplifier 204. Attached to slider pick-up 232 is one end of a pinion rack 205 whose opposite end is connected to a transparent member 206 on the surface of which is mounted correction curve 201, the latter 4being similar to correction curve |51 in Fig. 12. Immediately in front of the transparent member 236 is scale |23 which is mounted on a gear 203 meshing with gear 2|0. The latter meshes with gear 2|| attached to one end of shaft 2|2 whose opposite end carries a pinion 2|3 meshing with pinion rack 235. Intermediate transparent member 236 and scale |43 is transparent window |62 provided with a frosted surface |6|. Light source |58 and lens |53 are positioned rearwardly of transparent member 236. As the slider pick-up is moved along the conductors 20|, a portion 2|3 of correction curve 261 is projected onto the frosted surface |6| to constitute a moving index for the scale |113, similarly to the projected light `beam |63 in Fig. 12. In Fig. 16, it will be .understood that phase shifter 266 is merely illustrative of one device for obtaining variable phase shift by linear motion of a controlling slider pick-up '262, and alternate devices will be immediately suggested ,to those skilled in the art.

Fig. 17 shows another arrangement utilizing an optical device for providing a movable index for a calibrated scale, and comprises a phase shifter 223 mounted on one end of shaft 22| which also includes gear 222 and transparent disk 223 carrying a correction curve 226|, similar to correction curve |51 in Fig. 12. Meshing with gear 222 is a gear 225 attached rigidly to one end of shaft 226 whose opposite end carries a calibrated scale 221. Positioned adjacent the latter scale is a transparent window 228 having a frosted surface 229. Associated with the window 228 is a light source 236 and lens 23|. If desired, the scale 221 may include knob 232 connected to a clutch device, not shown, but similar to the clutch device associated with the scale |28 in Fig. 12 as previously described. In Fig. 17, scale 221 is similar to coarse scale |31 in Fig. 12 so that the movable index or projected light beam 233 in Fig. 17 on frosted surface 22S is similar to the projected light beam |63 in Fig. l2. Obviously, therefore, correction curves for the residual error of .the phase shifter may be associated with both the coarse and iine scales thereof where desired.

Fig. 18 shows an optical arrangement for correcting deviations from a predetermined scale which may or may not be linear and which may be of a type disclosed in the patent of T. Slonczewski No. 2,058,641 issued October 27, 1936. Referring to Fig. 18, a film 228 contains a predetermined scale 22| and a correction curve 222, similar to correction curve |51 in Fig. 12. Immediately behind the predetermined scale 24| is a transparent window 243 with a frosted surface 244. Associated with the foregoing is an optical system 245 which comprises a light source 246, lens 24'1, 90-d-egree prism 248, dove prism 249, and `EJO-degree prism '250. As shown in Fig. 18, the dove prism 299 is tilted at 45 degrees, thus rotating the beam of light passing therethrough by 90 degrees about the axis thereof. As a consequence, a portion 252 of correction curve 242 is projected onto the frosted surface 240 to intersect base line 20| of th-e predetermined scale 24| thereby constituting a movable index for 'the latter scale as the film 240 is moved. Obviously, the predetermined scale 24| and correction curve 242 may be formed on different film, disposed in different planes or at right angles, moving together; and alternate optical systems of wellknown types could be substituted for the optical system 245.

What is claimed is:

l. The method of measuring relative phase shift between two parallel signal transmission Vpaths for establishing a predetermined point of phase shift in one path and thereby a predetermined absolute standard of phase shift, which comprises transmitting an alternating current signal of preselected frequency through said two paths, selecting an arbitrary point of phase shift for said one path, introducing such amount of phase shift in said second path as to detect and measure zero relative phase shift between -said two paths, introducing approximately a predetermined amount of additional phase shift in said second path and adjusting the phase shift in said one path to detect and measure zero relative phase shift between -said two paths whereby a point representing the approximately predetermined amount of additional phase shift is established in said one path, removing the approximately predetermined amount of additional phase shift from said second path and adjusting the phase shift in said latter path to `detect and measure the zero relative phase shift between said two paths, reintroducing the approximately predetermined amount of additional phase shift in said second path and further adjusting the phase shift in lsaid vone path to detect and measure the zero relative phase shift between said two paths whereby said arbitrary point of phase shift for said one path is approximately reestablished therein, and readjusting the magnitude of the approximately predetermined amount of additional phase shift reintroduced in said second path in a direction toward the predetermined amount and repeating the aforementioned steps of detection and measurement of the zero relative phase shift rbetween said two paths for each of said last-'mentioned phase shift readjustments until said arbitrary point of phase shift is reestablished precisely in said one path, the reestablishment of said last-mentioned arbitrary point of phase shift in -said one path serving to establish precisely in said one path a point representing said predetermined amount of phase shift and at the same time to establish said predetermined amount of phase shift reintroduced in said second path as a predetermined absolute standard of phase shift.

2. The method of measuring relative phase shift between two parallel signal transmission paths for establishing points of or 360 and 180 phase shift in one path and thereby a 180 absolute standard of phase shift, which comprises transmitting an alternating current signal of preselected frequency through lsaid two paths, selecting arbitrarily said point of 0 or 360 phase shift for said one path, introducing such amount of phase shift in said second path as to detect and measure zero relative phase shift between said two paths, introducing approximately 180 additional phase shift in said second path and adjusting the phase shift in said one path to detect and measure the zero relative phase shift between said two paths whereby a point of 180 phase shift is approximately established in said one path, removing the approximately 180 additional phase shift from sai-d second path and further adjusting the phase shift in said latter path to detect and measure the zero relative phase shift between said two paths, reintroducing the approximately 180 additional phase shift in said second path and further adjusting the phase shift in said one path to detect and measure the zero relative phase shift between said two paths whereby said arbitrary point of 0 phase shift is approximately reestablished in said one path, and readjusting the magnitude of the approximately 180 additional phase shift reintroduced in said second path in a direction toward 180 and repeating the foregoing steps of detection and measurement of zero relative phase shift between said two paths for each of said last-mentioned phase shift readjustments until said arbitrary .point of 0 phase shift is precisely reestablished in said one path, the reestablishment of said last-mentioned arbitrary point of 0 phase shift in said one path serving to establish a precise 180 point of phase shift in said one path and simultaneously therewith to establish said 180 phase shift reintroduced in said second path as a 180 absolute standard of phase shift.

3. The method of measuring relative phase shift between two parallel signal transmission paths in accordance with claim 2 for establishing a point of phase shift in said one path thereby a 90 absolute standard of phase shift, which comprises selecting said arbitrary 0 point of phase shift in said one path, introducing such amount of phase shift in said second path to detect and measure Zero relative phase shift between said two paths, introducing approximately 90 additional phase shift in said second path and adjusting the phase shift in said one path to detect and measure the zero relative phase shift between said two paths whereby a point of 90 phase shift is approximately established in said one path, removing the approximately 90 additional phase shift from said second path and further adjusting the phase shift in said latter path to detect and measure the zero relative phase shift between said two paths, reintroducing the approximateiy 90 additional phase shift in said second path and further adjusting the phase shift in said one path to detect and measure the Zero relative phase shift between said two paths whereby the phase shift in said one path is reestablished approximately at said point, and readjusting the magntiude of the approximately 90 phase shift reintroduced in said second path in a direction toward 90 and repeating the nextmentioned steps of detection and measurement of zero relative phase shift between said two paths for each of said last-mentioned phase shift readjustments until said last-mentioned 180 phase shift point is precisely reestablished in said one path, the reestablishment of said last-mentioned 180 point of phase shift in said one path serving to establish precisely said point of 90 phase shift in said one path and at the same time to establish said additional 90 phase shift reintroduced in said second path as a 90 absolute standard of phase shift. 

